Reverse reaction sintering of Si3N4/SiC composites

ABSTRACT

A method of making a composite sintered silicon nitride/silicon carbide body, including mixing a predetermined amount of silicon nitride powder with a predetermined amount of silicon carbide powder, heat-treating the resultant mixed powder at a temperature of between about 800 and 1500 degrees Celsius in a substantially nitrogen sintering atmosphere, and producing a thin film of silica around individual silicon nitride and silicon carbide grains. The thin film of silica is useful in retarding the diffusion of oxygen to the silicon nitride particles, slowing their oxidation. The pressure of the sintering atmosphere is not substantially greater than atmospheric pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/597,049, filed Nov. 7, 2005.

BACKGROUND OF THE INVENTION

Polycrystalline silicon nitride (Si₃N₄) bodies are becoming increasinglyattractive as structural and mechanical materials due to their abilityto provide high strength and durability under severe conditions, andespecially under high temperature applications. Si₃N₄ is characterizedby high heat resistance, mechanical strength, thermal shock resistance,wear resistance, chemical stability, and hardness. One reason that Si₃N₄does not enjoy wider use is that Si₃N₄ green powder compacts or greenbodies are inherently difficult to sinter.

Known processes for producing a sintered silicon nitride body typicallyrequire the use of a sintering aid, such as Y₂O₃, Al₂O₃, MgO, or thelike, added to a raw material powder of silicon nitride and ahigh-pressure N₂ atmosphere or N₂/inert gas atmosphere under which theSi₃N₄ body is sintered. Typically, the sintering aids form a grainboundary liquid during sintering, and more typically this liquidincludes SiO₂, either contributed as an impurity present in the Si₃N₄ orgenerated by the oxidation of Si₃N₄ by oxygen present in the sinteringatmosphere.

The grain boundary liquid thus serves as a sintering aid and forms asilica-based glass in grain boundaries. This glass aids in thedensification of the silicon nitride powder green body and in theformation of a fine grain structure in the resulting sintered body.However, the relative amounts of O and N in the glass phase is anuncontrolled variable varies, and thus the composition of the glassyphase at the grain boundary is likewise uncontrolled and variable,resulting in density gradients in the sintered body and compositionalgradients at the grain boundaries.

Various additives have been added to improve the mechanical strength ofthe sintered ceramic bodies to enable them to perform under severeconditions. Silicon carbide (SiC) has been found to provide increasedresistance to oxidation and mechanical strength at high temperatures toSi₃N₄. However, sintered ceramics composites produced as described abovefrom a mixture of silicon nitride and silicon carbide powders typicallycontain silicon carbide particles on the order of microns only in thegrain boundaries of silicon nitride particles. Attempts have been madeto prevent the segregation of SiC in the grain boundaries of thesintered body. For instance, a composite sintered body of siliconnitride and silicon carbide was made by mixing a silicon nitride powderwith a fine silicon carbide powder having an average diameter of 0.03.mu.m and a specific surface area of 30 m.sup.2/g and an yttria to forma green body which was sintered at 1750-1900° C. in a pressurizednitrogen atmosphere of 1 MPa; the body was further subjected it to anHIP treatment at 1750° C. in a 100 MPa nitrogen atmosphere. However,such a technique requires a bimodal PSD in the starting mixture of Si₃N₄and SIC powders, and thus it is impossible to achieve a uniform mixture,resulting in a sintered body with an uneven grain structure. Further,the bimodal PSD of the main constituent powders makes an even, uniformdistribution of the yttria sintering aid unlikely, resulting ininsufficient sinterability and poor mechanical strength in the resultantsintered body.

Another technique for the production of a Si₃N₄/SiC body involves mixinga silicon metal powder, a silicon carbide powder and a sintering aidpowder, forming the mixture into a green body, sintering the green bodyin a nitrogen atmosphere to react the metallic silicon with nitrogen toform Si₃N₄ which functions to bond SiC particles, and then elevating thetemperature to further sinter the body via the sintering aid. Becausemetallic silicon is used instead of a silicon nitride powder as astarting material, shrinkage during sintering is minimized. However,this technique suffers from the difficulty in uniformly nitriding thesilicon metal from the surface to the core of the body, which typicallyresults in at least some silicon metal unreacted inside the resultingsintered body.

Yet another technique involves heat-treating a mixture of anorganosilicon polymer and silicon powder in a non-oxidizing atmosphere,such as N₂, and pulverizing it to form a silicacious powdercharacterized by a surface covered with an amorphous material consistingof silicon and carbon. The powder is formed into a green body andsintered in an N₂ atmosphere. However, as with the previously-descriedtechnique, it is difficult to uniformly nitride the body from surface tointerior.

Partially crystalline composite powders of silicon nitride and siliconcarbide have been produced as starting materials, mixed with a sinteringaid powder and formed into green bodies which have been heated to the1400-1600 degree Celsius range for a first sintering/reaction step andthen liquid phase sintered in the 1600-2300 degree Celsius range.However, the resulting sintered body typically suffers from thegeneration of pores and the deterioration of mechanical properties bydecomposition of amorphous components. Further, full density cannot beachieved via this technique absent elevated gas-pressure duringsintering. Also, since a sintering aid powder is mixed with thepartially crystalline composite powder and then sintered, the dispersionof the sintering aid powder is typically uneven, resulting insegregation of the sintering aid and inconsistent density and otherphysical properties observed in the sintered body.

Finally, a process of manufacturing a composite powder for themanufacture of a composite sintered body of silicon nitride and siliconcarbide includes the steps of mixing silicon metal powder andcarbonaceous powder together, heating the resultant mixture in an inertgas atmosphere, such as nitrogen, at a temperature of 1,400 degreesCelsius to simultaneously carbonize and nitride the silicon metalpowder. However, this technique suffers from the preferential formationof β-silicon nitride, making it difficult to increase the percentage ofα-silicon nitride in the composite powder. Since β-silicon nitride tendsto grow in a needle shape, the resultant powder is suffers from theanisotropic particle shapes and is thus difficult to compact orpulverize.

There thus remains a need for a technique for evenly sintering aSi₃N₄/SiC body to density that does not require a pressurized nitrogenatmosphere and/or excessively high firing temperatures, as bothrequirements greatly increase the expense of the process and, thus, theend product. The present invention addresses this need.

SUMMARY OF THE INVENTION

A silica film is produced around Si₃N₄ and SiC particles present in agreen body via oxidation at high temperature and is relatively thin andstable, allowing for control of oxidation of the particles duringsintering in air or other primarily nitrogen or inert gas mixture havinga relatively minor oxygen component, and under standard atmospheric orslightly to moderately elevated pressures. The production of a quantityof active oxide, allows for the avoidance of excessive oxidation and thesimultaneous sintering of the body. The guiding principle is one ofthermodynamics. Si₃N₄/SiC composites are thus feasible in terms ofthermodynamics. Better sintering can be achieved via controlling thetemperature ramp rate and atmosphere during the process of sintering. Atypical sintering temperature range is about 800 to about 1500 degreesCelsius; more typically, the sintering range is between about 800 andabout 1200 degrees Celsius, wherein control of oxidation of nitrogen andnitrogen-containing compounds may be maintained. Typically, the bodiesare soaked at about 1200 degrees Celsius to avoid excessivedecomposition of the nitride. Sintering below the high temperature of1500 degrees Celsius allows for the so-produced compact Si₃N₄/SiCcomposite of better quality.

The microstructure of products which made by sintering SiC—Si₃N₄ blendsin the above temperature ranges show:

1. The main composition of the matrix of SiC—Si₃N₄ system aftersintering are SiO₂ and Si₂N₂O, and the Si₃N₄ in the matrix is mostlydecomposed; however, the compact strength is still high.

2. The main composition of the matrix of SiC—Si₃N₄—Si system aftersintering are SiO₂, Si₂N₂O and metallic silicon, the crystal morphologyof Si₂N₂O is fine and small, and the ratio of remnant nitrogen ishigher.

3. The main composition of the matrix of SiC—Si₃N₄—SiO₂ system aftersintered are SiO₂ and Si₂N₂O. The Si₂N₂O crystals on the surface growbetter, but Si₂N₂O crystals in the interior are still present, butsmaller. The compact strength is high and the ratio of remnant nitrogenis higher.

One object of the present invention is to provide a method for sinteringSiC—Si₃N₄ at ambient pressure and in air. Related objects and advantagesof the present invention will be apparent from the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration of a plot of standard free energy vs.absolute temperature for several oxidation reactions within thesilicon-carbon-oxygen-nitrogen system.

FIG. 2 is a graphical illustration of a plot of standard free energy vs.absolute temperature for several other oxidation reactions within thesilicon-carbon-oxygen-nitrogen system.

FIG. 3 is a phase diagram of the Si—N—O system at 1800 Kelvin

FIG. 4 is a graphical illustration of a plot of standard free energy vs.absolute temperature for several sintering/oxidation reactions withinthe silicon-carbon-oxygen-nitrogen system.

FIG. 5 is a graphical illustration of a plot of standard free energy vs.Celsius temperature for several regular non-oxide materials.

FIG. 6 is an X-Ray diffraction pattern taken of the as-sintered surfaceof SiC/Si₃N₄ composite sample P1 prepared according to a firstembodiment of the present invention.

FIG. 7 is an X-Ray diffraction pattern taken of the interior of sampleP1.

FIG. 8 is an SEM photomicrograph of sample P1.

FIG. 9 is a graphic representation of an n ESA analysis of sample P1.

FIG. 10 is an X-Ray diffraction pattern taken of the as-sintered surfaceof SiC/Si₃N₄ composite sample P2 prepared according to a firstembodiment of the present invention.

FIG. 11 is an X-Ray diffraction pattern taken of the interior of sampleP2.

FIG. 12 is a first SEM photomicrograph of the surface region of sampleP2.

FIG. 13 is a second SEM photomicrograph of a pore of sample P2.

FIG. 14 is a first SEM photomicrograph of the interior of sample P2.

FIG. 15 is a second SEM photomicrograph of the interior of sample P2.

FIG. 16 is an X-Ray diffraction pattern taken of the as-sintered surfaceof SiC/Si₃N₄ composite sample P3 prepared according to a firstembodiment of the present invention.

FIG. 17 is an X-Ray diffraction pattern taken of the interior of sampleP3.

FIG. 18 is a first SEM photomicrograph of the surface region of sampleP3.

FIG. 19 is a second SEM photomicrograph of a surface region of sampleP3.

FIG. 20 is a first SEM photomicrograph of the interior of sample P3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention and presenting its currently understood best mode ofoperation, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, with such alterations and furthermodifications in the illustrated device and such further applications ofthe principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

Oxidation of SiC

When SiC particles are exposed to an oxygen-containing environment atelevated temperatures, a surface film of SiO₂ may form on the SiCparticles through the partial oxidation of the SiC. The SiO₂ film actsas a protective film preventing the complete oxidation of the SIC,since, unless the environment is very rich in oxygen, oxygen diffusionthrough the silica film is very slow, even at elevated temperatures. Asthe system typically contains, in addition to silicon carbide, metalsilicon, carbon and silica, the actual reaction kinetics are influencedby the relative amounts of C, SiC, Si, and SiO₂.

In the above system, SiC is decomposed into SiO (g), and SiO (g) isfurther oxidized into SiO₂(s). The speed of decomposition and oxidationof SiC is a function of the composition of temperature and the sinteringatmosphere. The reaction may be described thermodynamically as follows.SiC(s)+O₂(g)=Si(s)+CO₂(g)Δ_(r) G ^(θ) ₁=−318560−12.1{T} _(K)(J·mol⁻¹)  2.12SiC(s)+O₂(g)=2Si(s)+2CO(g)Δ_(r) G ^(θ) ₂=−72772−196.2{T} _(K)(J·mol⁻¹)  2.22/3SiC(s)+O₂(g)=2/3SiO(g)+2/3CO₂(g)Δ_(r) G ^(θ) ₃=−284610−59.4{T} _(K)(J·mol⁻¹)  2.32SiC(s)+O₂(g)=2SiO(g)+2C(s)Δ_(r) G ^(θ) ₄=−64728−176.2{T} _(K)(J·mol⁻¹)  2.4SiC(s)+O₂(g)=SiO(g)+CO(g)Δ_(r) G ^(θ) ₅=−144740−175.1{T} _(K)(J·mol⁻¹)  2.5SiC(s)+O₂(g)=SiO₂(s)+C(s)Δ_(r) G ^(θ) ₆=−832680+166.6{T} _(K)(J·mol⁻¹)  2.62/3SiC(s)+O₂(g)=2/3SiO₂(s)+2/3CO(g)Δ_(r) G ^(θ) ₇=−630033−53.1{T} _(K)(J·mol⁻¹)  2.71/2SiC(s)+O₂(g)=1/2SiO₂(s)+1/2CO₂(g)Δ_(r)G^(θ) ₈=−613600+82.8{T} _(K)(J·mol⁻¹)  2.82C(s)+O₂(g)=2CO(g)Δ_(r) G ^(θ) ₉=−224760−174.1{T} _(K)(J·mol⁻¹)  2.92CO(g)+O₂(g)=2CO₂(g)Δ_(r) G ^(θ) ₁₀=−564350+172{T} _(K)(J·mol⁻¹)  2.102Si(s)+O₂(g)=2SiO(g)Δ_(r) G ^(θ) ₁₁=−216710−154.1{T} _(K)(J·mol⁻¹)  2.112SiO(g)+O₂(g)=2SiO₂(s)Δ_(r) G ^(θ) ₁₂=−1600600+509.5{T} _(K)(J·mol⁻¹)SiC(s)=Si(s)+C(s)Δ_(r) G ^(θ) ₁₃=75992−11.1{T} _(K)(J·mol⁻¹)  2.13

It is evident that the value of Δ_(r)G^(θ) in reaction 2.13 is positivebetween 1000 and 1800 K, and thus the reaction cannot proceed. FIG. 1 ismade according to the relation of Δ_(r)G^(θ) and temperature in thescope of 1000 and 1800 K. Regarding FIG. 1, it can be seen that underthe standard state or at an oxygen partial pressure of 0.1 Mpa, thevalue of □Δ_(r)G^(θ) in all reactions within the scope of temperatureare negative, so the reactions can proceed. Accordingly, the value ofΔ_(r)G^(θ) in reaction 2.12 is the most negative, and those in reactions2.6, 2.7, 2.8, and 2.9 also relatively large negative values; thus theoxidation of SiC will proceed according to these reactions, the productsof which are mainly SiO₂ and the gases CO₂ and CO.

The oxidation of SiC is influenced by many factors, such as the reactiontemperature, sintering atmosphere, the composition of binding agent andcrystal structure, diffusion speed of the various species taking part inthe reactions, the particle size of the SiC, the particle sizedistribution, and the like. The oxidation of SiC may be envisioned as amulti-phase reaction, with O₂ as a diffusing phase and SiC as a fixedphase. The oxidation process is thus dependent on the diffusion of O₂ tothe SiC and the reaction of O₂ at the SiC interface. Assuming a bodyformed primarily of SiC particles, the oxygen arrives at the SiCparticle surface by a diffusion mechanism and reacts, forming the filmof SiO₂. Once the silica film is established, additional oxygen mustpass through the silica film to reach the SiC interface to further reacttherewith; thus, the diffusion of oxygen through the silica film becomesrate limiting. Meanwhile, the gaseous species CO₂, CO and/or SiO thatare likewise produced must pass through the silica film. In other words,CO₂, CO and/or SiO are emitted by reverse diffusion, which alsoinfluence the diffusion of oxygen to the SiC interface. Thus, beyond theinitial forming of the silica film, the rate of the oxidation of SiC ismainly controlled by diffusion of the various gaseous species throughthe silica film. Thus the oxidation speed has close relationship withthe structure of product layers, the degree of compaction of the body,the oxidation temperature and time, and the property of the diffusionmatter. Additionally, the multi-crystal transformation from α-silica toβ-silica also impacts the diffusion of O₂ therethrough as well as havingan effect on the compaction of the body, thus influencing the oxidationrate of the SiC particles.

Oxidation of Si₃N₄

Si₃N₄ is a non-oxide, and is thus easily oxidized at high temperatures.The oxidation behavior of Si₃N₄ and its products are dependent upon theambient oxygen partial pressure: under high oxygen partial pressures,the oxidation products of Si₃N₄ are mainly SiO₂ and N₂, along with smallamounts of Si₂N₂O (g) and NO (g); under low oxygen partial pressures,the main oxidation products are solid SiO₂ and gaseous SiO, with smallamounts of N₂ (g). According to thermodynamics, the oxidation reactionsof Si₃N₄ under high temperatures may be expressed as follows:1/3Si₃N₄(s)+O₂(g)=SiO₂(s)+2/3N₂(g)Δ_(r) G ^(θ) ₁=−657533+64.1{T} _(K)(J·mol⁻¹)  2.141/5Si₃N₄(s)+O₂(g)=3/5SiO₂(s)+4/5NO(g)Δ_(r) G ^(θ) ₂=−322260+28.4{T} _(K)(J·mol⁻¹)  2.152/3Si₃N₄(s)+O₂(g)=2SiO(g)+4/3N₂(g)Δ_(r) G ^(θ) ₃=285360−381.3{T} _(K)(J·mol⁻¹)  2.162/7Si₃N₄(s)+O₂(g)=6/7SiO(g)+8/7NO(g)Δ_(r) G ^(θ) ₄=225606−177.8{T} _(K)(J·mol⁻¹)  2.174/3Si₃N₄(s)+O₂(g)=2Si₂N₂O(s)+2/3N₂(g)Δ_(r) G ^(θ) ₅=−842547+38.2{T} _(K)(J·mol⁻¹)  2.184/5Si₃N₄(s)+O₂(g)=6/5Si₂N₂O(s)+4/5NO(g)Δ_(r) G ^(θ) ₆=−424760+13{T} _(K)(J·mol⁻¹)  2.192/3Si₂N₂O(s)+O₂(g)=4/3SiO₂(s)+2/3N₂(g)Δ_(r) G ^(θ) ₇=−602080+79.6{T} _(K)(J·mol⁻¹)  2.20SiO₂(s)=Si(s)+O₂(g)Δ_(r) G ^(θ) ₈=908670−177.7{T} _(K)(J·mol⁻¹)  2.21Si₃N₄(s)=3Si(s)+2N₂(g)Δ_(r) G ^(θ) ₉=753100−340.8{T} _(K)(J·mol⁻¹)  2.222Si₂N₂O(s)=4Si(s)+2N₂(g)+O₂(g)Δ_(r) G ^(θ) ₁₀=1834100−488.1{T} _(K)(J·mol⁻¹)  2.23

FIG. 2 expresses the relationship of Δ_(r)G^(θ) in the above reactionsand temperature between 1000 and 1800 K. It can be seen in FIG. 2 thatunder the standard state or at the oxygen partial pressure of 0.1 Mpa,the value of Δ_(r)G^(θ) in reaction 2.21, 2.22, and 2.23 are allpositive and thus the three reactions cannot proceed. Reaction 2.17 mayproceed when the temperature exceeds 1269 K; the value of Δ_(r)G^(θ) inthe other reactions in this temperature range are all negative, and thusreactions 2.19 and 2.20 may proceed. Under high oxygen partialpressures, the oxidation of Si₃N₄ can occur basically according to theabove four reactions, with the products being mainly SiO₂ and N₂. If theoxygen partial pressure is in the lower range, the gases of Si₂N₂O andNO may likewise be produced in significant quantities. The relationshipof condensed phase and gas phase in the Si—N—O system and the balancedoxygen partial pressure in gas phase may be expressed as follows:Si(s)+O₂(g)=SiO₂(s)1 g(p _(O2) /p ^(θ))=−22.449  2.243Si(s)+2N₂(g)=Si₃N₄(s)1 g(p _(N2) /p ^(θ))=−4.341  2.254Si₃N₄(s)+3O₂(g)=6Si₂N₂O(s)+2N₂(g)1 g(p _(N2) /p ^(θ))=3/31 g(p _(O2) /p ^(θ))+37.616  2.262Si₂N₂O(s)=4Si(s)+2N₂(g)+O₂(g)1 g(p _(N2) /p)=−1/21 g(p _(O2) /p ^(θ))−18.326  2.272Si₂N₂O(s)+3O₂(g)=4SiO₂(s)+2N₂(g)1 g(p _(N2) /p ^(θ))=3/21 g(p _(O2) /p ^(θ))+26.572  2.28Si₃N₄(s)+3O₂(g)=3SiO₂(s)+2N₂(g)1 g(p _(N2) /p ^(θ))=3/21 g(p _(O2) /p ^(θ))+29.333  2.29From the above relationships, it can be seen that the reaction in theinterface of Si₃N₄ and SiO₂ may proceed as follows, producing Si₂N₂O(FIG. 3):4Si₃N₄(s)+3O₂(g)=6Si₂N₂O(s)+2N₂(g)or Si₃N₄(s)+SiO₂(s)=2Si₂N₂O(s)Oxidation of Si₂N₂O

Theoretically, Si₂N₂O may be totally oxidized under an oxidizingatmosphere (without the protection of the silica film) if the reactionis allowed to proceed for a sufficiently long time. Under the oxidizingatmosphere (i.e., a high oxygen partial pressure), the product ofoxidation is mainly SiO₂ and the gas phase is mainly N₂ withoutsubstantial SiO gas. But under low oxygen partial pressure, the gasphases in the products of oxidation are mainly SiO gas and a little N₂.The above thermodynamics calculation results of equations 2.16 and 2.29can be seen and as following equation 2.30:1/3Si₃N₄(s)+O₂(g)=SiO₂(s)+2/3N₂(g)Δ_(r) G ^(θ)=−663776.7+69.31{T} _(K)(J·mol⁻¹)  2.30

To sum up, the oxidation of Si₃N₄ can produce the different resultsaccording to the different temperature and oxygen partial pressureconditions. At high temperature and under an oxidizing atmosphere, theoxidation reactions of Si₃N₄ are as follows:Si₃N₄(s)+3O₂(g)=3SiO₂(s)+2N₂(g)4Si₃N₄(s)+3O₂(g)=6Si₂N₂O(s)+2N₂(g) and2Si₂N₂O(s)+3O₂(g)=4SiO₂(s)+2N₂(g)So the above reactions can proceed at high temperature and underoxidizing conditions. At high temperature and under low oxygen partialpressure, the oxidation reaction of Si₃N₄ is as follows:Si3N4(s)+3/2O2(g)=3SiO(g)+2N2(g)In addition to this, there is also4Si3N4(s)+3O2(g)=6Si2N2O(s)+2N2(g)which should be avoided during the sintering process.The Si3N4/SiC System

In the Si₃N₄/SiC multi-phase system, in addition to above oxidationreactions of SiC and Si₃N₄, other reactions may occur, which are asfollows:SiO₂(s)+CO(g)=SiO(g)+CO₂(g)Δ_(r) G ^(θ) ₁=517960−168.7{T} _(K)(J·mol⁻¹)  2.311/6Si₃N₄(s)+CO(g)=1/2SiO₂(s)+1/3N₂(g)+CΔ_(r) G ^(θ) ₂=−21643+119.1{T} _(K)(J·mol⁻¹)  2.321/2SiC(s)+CO(g)=1/2SiO₂(s)+3/2C(s)Δ_(r) G ^(θ) ₃=−304280+171.2{T} _(K)(J·mol⁻¹)  2.33SiC(s)+CO₂(g)=SiO₂(s)+2C(s)Δ_(r) G ^(θ) ₄=−437210+167.1{T} _(K)(J·mol⁻¹)  2.341/3Si₃N₄+CO₂(g)=SiO₂+C+2/3N₂(g)Δ_(r) G ^(θ) ₅=−262+64.5{T} _(K)(J·mol⁻¹)  2.351/2SiO₂(s)+SiO(g)+N₂(g)=1/2Si₃N₄(s)+O₂(g)Δ_(r) G ^(θ) ₆=186140+158.6{T} _(K)(J·mol⁻¹)  2.363/2SiC(s)+N₂(g)=1/2Si₃N₄(s)+3/2C(s)Δ_(r) G ^(θ) ₇=−262565+153.8{T} _(K)(J·mol⁻¹)  2.373/2SiO₂+N₂(g)=1/2Si₃N₄+3/2O₂(g)Δ_(r) G ^(θ) ₈=984600−95.3{T} _(K)(J·mol⁻¹)  2.38

FIG. 4 illustrates the relationship of Δ_(r)G^(θ) in above reactions andtemperature between 1000 and 1800 K. FIG. 4 shows that under thestandard state or at high oxygen partial pressure, only the value ofΔ_(r)G^(θ) in reaction 2.34 within this temperature range is negative,and thus may proceed. Reaction 2.33 may proceed when the temperature isbelow 1778 K, and reaction 2.37 may proceed when the temperature isbelow 1707 K. For other five reactions, Δ_(r)G^(θ) is positive in thetemperature range from 1000 to 1800 K, and thus the reactions cannotproceed. In other words, from the point of view of thermodynamics, SiCmay possibly be reacted with CO₂ and a little CO to produce SiO₂ and C;SiC may also be reacted with N₂ (produced by the oxidation of Si₃N₄) toproduce Si₃N₄ and C. Certainly, C thus produced may be further oxidizedinto CO₂ and CO under the oxidizing atmosphere.

The Oxidizing Sequence of Si₃N₄ and SiC when Sintered.

Referring to FIG. 5, it can be seen that Si₃N₄ will be oxidized beforeSiC within the sintering temperature range. As discussed above, Si₃N₄reacts with oxygen as expressed in reactions 2.30, 2.26 and 2.16, whichmay proceed depending on the oxygen partial pressure. From FIG. 3, itcan be seen that when p_(O) ₂ is relatively low, Si₂N₂O is firstproduced, and the reaction product is SiO₂ with rising p_(O) ₂ .Regarding SiO, because it is a gaseous compound, the discussion may becarried out through the relationship of SiO and SiO₂.

${{SiO}(g)} + {\frac{1}{2}{O_{2}(g)}\mspace{14mu}{{SiO}_{2}(s)}}$Δ_(r) G ^(θ)=−812140−325.18{T} _(K)(J·mol⁻¹)  2.39Δ_(r) G=Δ _(r) G ^(θ) −RT ln(p _(SiO) /p ^(θ))·(p _(O) ₂ /p^(θ))^(1/2)  2.40

When p_(O) ₂ is higher, the negative value is increased, Δ_(r)G<0 andthe reaction can be carried out toward the direction of producing SiO₂;when p_(O) ₂ is lower, Δ_(r)G>0 and SiO is produced. SiO is a gaseouscompound and, once produced, can be volatilized out, especially whenp_(O) ₂ is lower than convertible oxygen partial pressure. Thus, SiO canreach the outside pressure and be volatilized rapidly. In this process,SiO₂ is expected as the produced surface layer and SiO₂ or Si₂N₂O are atthe inner particulate interface; SiO is not expected to form and bevolatilized. Thus, the reactions must be carried out under higher p_(O)₂ .

The Stability of the Surface Film of SiO₂

SiO₂ is in the compact contacting state with Si₃N₄ and SiC, so thereaction between them must be considered.1/3Si₃N₄+SiO₂=2SiO(g)+2/3N₂(g)Δ_(r) G ^(θ)=931363−442.25{T} _(K)(J·mol⁻¹)  2.41Δ_(r) G=Δ _(r) G ^(θ) +RT ln(p _(SiO) /p ^(θ))²·(p _(N) ₂ /p^(θ))^(2/3)  2.42From reaction 2.44, it can be shown that 1 mol N₂ and 3 mol SiO areproduced at same time, that isp _(SiO) /p ^(θ)=3p _(N) ₂ /p ^(θ) andΔ_(r) G=Δ _(r) G ^(θ)+2RT ln 3+RT ln(p _(N) ₂ /p ^(θ))^(8/3)  2.43andΔ_(r) G=931363−423.98T+RT ln(p _(N) /p ^(θ))^(8/3)  2.44are thus obtained.

If the production of gaseous SiO and N₂ is sufficient to break throughthe surface film of SiO₂, the sum of pressure of the two gases should bemore than the atmospheric pressure, that is:p _(SiO) /p ^(θ) +p _(N) ₂ /p ^(θ)=1 and p _(SiO) /p ^(θ)=3p _(N) ₂ /p^(θ)  2.45So p _(N) ₂ /p ^(θ)=¼ is obtained andΔ_(r) G=931363−454.71T  2.46is obtained.

Thus the transition temperature of T=2048.26 K=1775.03° C. can beobtained. That is, when the temperature is higher than this value, thegases of SiO and N₂ can break through the surface film of SiO₂, and thecomposite material cannot be protected. But if the sintering temperatureof the process is lower than 1500° C., the surface film produced canprotect the so-coated particle.

Because the surface film is covered on the composite material, it isalso influenced by SiC. Thus the expressionSiC+2SiO₂=3SiO(g)+CO(g)  2.47wherein the transition temperature of T=2048 K=1775° C. is obtained,which indicates the surface film of SiO₂ can protect the compositematerial.

Another situation regarding the effect of the producing Si₂N₂O islikewise considered. When SiC exists, the oxygen partial pressure in thefilm is lower, and the possibility of producing Si₂N₂O is greater.2/3Si₃N₄+SiO₂=Si₂N₂O+SiO(g)+1/3N₂(g)Δ_(r) G ^(θ)=375332−236.06T  2.48Δ_(r) G=Δ _(r) G ^(θ) +RT ln(p _(SiO) /p ^(θ))·(p _(N) ₂ /p^(θ))^(1/3)  2.49

The reaction here is also that 1 mol N₂ and 3 mol SiO are produced atthe same time, so p_(SiO)/p^(θ)=3p_(N) ₂ /p^(θ). The pressure which canbreak through the surface film is greater than atmospheric pressure,that is p_(SiO)/p^(θ)+p_(N) ₂ /p^(θ)=1 and p_(N) ₂ /p^(θ)=¼ is obtained,and thusΔ_(r) G=375332−241.46T  2.50

The transition temperature of T=1554.45 K=1281.22° C. is obtained. Thatis, when Si₂N₂O is produced, the gas pressure of SiO and N₂ produced canbreak through the surface film at a relatively low temperature. But theSi₂N₂O can still form the protecting film. If the oxygen partialpressure is high enough to oxidize Si₂N₂O, then the protecting SiO₂ filmis formed. This is also the basis of forming a Si₂N₂O/Si₃N₄/SiC system.

The Sintering Process of Reverse Reaction Sintering Si₃N₄/SiC Composites

The sintering process of reverse reaction sintering Si₃N₄/SiC compositesis actually the reaction process of controlling oxidation, by reactingto produce new and active SiO₂ and a little Si₂N₂O which segregates ontothe SiC and Si₃N₄ particle surfaces, thus aiding in sintering.Additionally, the presence of impurities, to some extent, may beregarded sintering aids; for example clay, metal Si, SiO₂, and the like,may further assist in sintering. The oxidation of reverse reactionsintering Si₃N₄/SiC composites is a function of such factors as thereaction temperature, the sintering atmosphere, the composition ofbinding agent and crystal structure, the particle size of raw material,the particle size distribution of the system, and the like. In thisprocess, O₂ is a diffusing phase, SiC and Si₃N₄ are solid phases, andthe oxidation process is limited by the diffusion of O₂ to the particleinterfaces and the diffusion of reaction products away therefrom. Oxygendiffuses to the surface of the SiC and Si₃N₄ particles and forms a filmof SiO₂ thereupon. To further react with a given particle, oxygen mustfirst diffuse through the SiO₂ film to reach the reaction interface; theoxidation reaction is thus limited by the diffusion of oxygen throughthe silica shells that form on the respective particles. Likewise, thegaseous reaction products, such as CO₂, CO, SiO, N₂ and NO, and thelike, are emitted by reverse diffusion from the interface through thesilica layer, which also influences the diffusion of oxygen therethroughto the interface.

The driving force of sintering is the surface energy (surface tension).The powder material is typically highly dispersed, and more typically ischaracterized by an extremely large specific surface area, and thus hasrelatively high surface energy. As systems have a tendency to achievethe state of lowest energy, the reduction of surface free energy is themain driving force of sintering the material. The difference between thesurface energy (ε_(b)) of powder particles and the interface energy(ε_(s)) of crystal particles of multi-crystal sinter will result in thereduction of free energy of system, and the ratio ε_(b)/ε_(s) is thus ameasure of the sintering character of powder.

The first step of sintering process can be regarded as the compacting ofthe body formed of partially compacted particulate material, such as amixture of SiC and S₃N₄ particles. During the initial period (whereinthe temperature is below 800 degrees Celsius), the body is heated in theair and a thin silica layer is formed around substantially all of theparticles; the silica layer thus prevents further oxidation of thesiliceous particles. As the temperature is increased, impurities in theraw material may be reacted with SiO₂ to produce a lower melting pointeutectic material. Typically, the viscosity of eutectic liquid isrelatively low and the particles making up the body may be redistributedby surface tension. The second step of sintering process is typicallyone of dissolving-diffusing-reseparating out.

During this step, the diffusion of O₂ through the silica layer limitsthe reaction rates. Due to the existence of the eutectic liquid, thespeed of compacting the body is increased. After the particles areredistributed, they are separated by the thin liquid film. As the bodydensifies, the liquid separating the particles becomes quite thin.Typically, the thinner the liquid film, the greater the pressure of theparticles. The solubility at the point of particles contacting isincreased due to this pressure. The material at the contact points isgradually dissolved into the liquid, and then transferred to othersurface and separated out.

The third step of sintering process is the process of grain growth. Dueto the shrinkage/disappearance/closure of the pores, sintering speed isreduced but the microstructure of the material still continues tochange. That is, other phenomena such as the grain growth, necking, andcapillary action of liquid filing pores still continue to occur, but atslower rates. During the cooling process, the remaining interfacialliquid is hardened to glassy state or partially crystallized. If anexterior force is applied, the degree of compacting between theparticles may be accelerated. As the thermodynamic equations illustrate,the sintering process is accompanied by the oxidation of Si₃N₄, whichprovides a continuous emission of N₂.

The Microstructure Analysis of Reverse Reaction Sintering Si₃N₄/SiCComposites

The study on microstructure of reverse reaction sintering Si₃N₄/SiCcomposites includes the study on the structure of micro minerals and ofmicro pores. The surface area and inner area of sample P1, P2, P3 and P4were analyzed by XRD, SEM and EPMA in order to confirm themicrostructure of the sintered samples. The result of the analyses areas follows:

The Study on Microstructure of SiC—Si₃N₄ Sintering System

Particle Sample # P1 Raw Materials Size(mm) (weight %) Sample # P2Sample # P3 SiC 2.8-0.9  35 35 30 0.9-0.15 30 30 30 0.115 5 5 5 0.063 50.045 10 10 10 Si₃N₄ 0.088 15 15 15 SiO₂ 10 Si 5

Sample P1 was analyzed by XRD and SEM. FIGS. 6 and 7 are surface area(0-5 mm) and inner area (8-16 mm) XRD patterns, respectively. Thesamples were prepared by mixing the constituent powders and forming theminto green bodies. This was done by pressing at about 105 MPa; somesamples required small amounts of binder (dextrine solution) to allowpressing. The green bodies were dried at 105 degrees Celsius for 10hours. The green bodies were then heated at a rate of 50 degreesCelsius/hour to about 800 degrees Celsius, where they were allowed tosoak for 8 to 10 hours. The samples were then heated at a rate of 50degrees Celsius/hour to about 1450 degrees Celsius, where they wereallowed to soak for 5 hours. The samples were then cooled to roomtemperature. All sintering was done in air under normal atmosphericpressures.

FIG. 6 shows that the surface sample of P1 is one of 0-5 mm area and itsmain crystal phases are SiC, Si₃N₄ and SiO₂. FIG. 7 shows that theinterior of sample of P1 is mainly composed of SiC, Si₃N₄ and SiO₂, butwith more Si₃N₄ and less SiO₂ than at the surface. Thus, the sinteringand oxidation of the system without a sintering agent yields SiC andSi₃N₄ dispersed in a primarily silica matrix.

FIGS. 8 and 9 relate to a nitrogen-containing oxide found in the sectionof sample P1 and its morphology. SEM and ESA show the existence ofnitride in the state of conglomerate, indicating that Si₂N₂O is producedduring the oxidation of Si₃N₄. The absence of the primary peak forSi₂N₂O in XRD pattern indicates that either the amount of Si₂N₂O presentis relatively small, the Si₂N₂O is amorphous or glassy, or the Si₂N₂Ocrystal structure includes sufficient impurities so as to besubstantially distorted.

Thus, if no any sintering agent is added when the SiC—Si₃N₄ powder issintered, the main oxidation products produced during sintering are SiO₂and a little Si₂N₂O. Thus, the oxidation sintering reaction of thesystem progresses and Si₃N₄ is substantially oxidized. Typically, theamount of SiO₂ in the matrix of system is kept below a predeterminedthreshold value, since excess silica can degrade the durability of thesintered body. Erosion testing of the P1 sample in cryolite-sodiumfluoride melt in an electrobath of aluminum demonstrated that the amountof silica was not in excess, as the sintered P1 sample still had theproperty of preventing penetration and melt wetting resistancecharacteristic of sintered Si3N4/SiC composite materials.

The Study on Microstructure of SN₄—SiC—Si Sintering System

Sample P2 was analyzed by XRD and EPMA; the results are presented asFIGS. 10 and 11, and are of the surface (0-5 mm) and interior area (8-16mm), respectively. As can be seen from FIGS. 10 and 11, metallic siliconis present in the sintered matrix of P2. The oxidation products ofsintering are Si₂N₂O and a little SiO₂; the main crystal phases arehexagonal SiC and Si₃N₄. But compared with the X-ray patterns of thesurface and inner area of P2, it can be seen that the content of crystalphase of Si₂N₂O and SiO₂ in the surface area (FIG. 10) of the sample arerelatively high (compared with the peak strength). Metallic siliconstill exists in the surface area (FIG. 10). There is relatively littleSi₂N₂O crystal phase present in the inner area (FIG. 11) is little.

Electron microprobe analysis results for sample P2 are shown in FIGS. 12and 13. Turning to FIG. 12, the surface area of P2 the distribution ofmetallic silicon, with the diameter of the silicon particles being lessthan about 50 μm; the metallic silicon particles are distributed in theamong of coarse SiC particles, which indicates that metallic silicon inthe surface area has not been disappeared totally after being sinteredat the relatively high temperature of 1450 degrees Celsius. In addition,there is obvious chromatic aberration in the surface area along thepores extending below the surface (see FIG. 13). Comparison of thedistribution of O, C and N indicates that the surface of the particleshave an area of high oxygen content. Thus, the reaction of oxygendiffusing toward surrounding area occurs around the pores.

FIG. 14 illustrates the interior area (8-16 mm) morphology of sample P2,and shows that the content of metallic silicon in the inner area isrelatively greater and concentrated in the voids of SiC. Suchdistribution appears to be beneficial to the oxidation resistance of SiCand thus improves the binding strength. Apparently, metallic silicon ismelted into the SiC voids and acts as a binder.

FIG. 15 illustrates the Si₂N₂O mineral morphology in inner area (8-18mm) of sample P2. Significant Si₂N₂O is present in the interior area ofsample P2. Si₂N₂O is present on the surface of grains Si₃N₄ as short,cylinder crystals. Further, silica has precipitated in the surroundingarea. Inner oxidation of pores is also be observed, and the oxidationprocess extends toward the sample interior along the void surface area.SiO produced by oxidation is precipitated onto the surface due to totaloxidation of exterior surface, which prevents further oxidation. TheSi₂N₂O crystal morphology is not as obvious, and the Si₂N₂O crystalsseem to grow finer. So the ratio of remnant nitrogen is high and thestrength of the sintered P2 sample was high as well. The X-ray patternconfirms the presence of Si₂N₂O. And it is probable that the oxygendiffused into the matrix reacts preferentially with metallic silicon.

So, the metal silicon (surface and inner area) in the sample matrix isnot totally oxidation during the sintering of the SiC—Si₃N₄ sample. Themetallic silicon apparently infiltrates into the void of the SiCcrystals at the sintering temperature and acts in a binding role, likelyas a plastic phase. Meanwhile, metallic silicon reacts more easily withoxygen than does Si₃N₄, so the formation of a Si₂N₂O phase is notobserved, and thus more Si₃N₄ is present after the sample is sintered.The sample thus produced has better chemical durability characteristics,which were confirmed by the erosion test of cryolite-sodium fluoridemelt test.

The Study on Microstructure of Si₃N₄—SiC—SiO₂ Sintering System

Sample P3 was analyzed by XRD and EPMA. FIGS. 16 and 17 show theexterior surface area and inner area XRD pattern for sample P3,respectively. Sample P3 was produced by sintering SiC—Si₃N₄ in thepresence of SiO₂. The analysis sample P3 shows that the main phases areSiC and Si₃N₄, but the content of Si₂N₂O in the surface area of thebinding phase is less than that of the inner area (comparison of peakstrength). The SiO₂ phase in the surface area is relatively high(comparison of peak strength) compared to that of the inner area(significant amounts of SiO₂ were not observed in X-ray pattern). Inaddition, the primary peak of Si₂N₂O mineral was detected both onsurface and interior of the P3 sample, indicating that the Si₂N₂Ocontent was higher and SiO₂ may be present in an amorphous or glassystate; if so, the structure is suited for use in a thermal shockresistance environment.

FIG. 18 is surface EPMA pattern of sample P3; FIG. 19 is surface areaSi₂N₂O morphology of sample P3; and FIG. 20 is interior area (8-16 m)morphology of sample P3. The analysis of FIG. 18 shows that theoxidation product of Si₃N₄ on the surface is Si₂N₂O and SiO₂, and thecrystal grows very well (seeing FIG. 19). Meanwhile, Si₃N₄ here may becovered by SiO₂ and Si₂N₂O and thus cannot be detected. FIG. 20 showsthat Si₂N₂O crystal from the oxidation of Si₃N₄ are very fine; this maybe an effect of the speed of gas diffusion through the silica layer.

To sum, by sintering the product of SiC—Si₃N₄ in the presence of SiO₂,the process of oxidation in the surface area is relatively smooth, asthe oxidation of Si₃N₄ is carried out in the presence of a superfineSiO₂ layer. The existence of the superfine SiO₂ layer reduces orprevents the gas diffusion, and makes the oxidation of Si₃N₄ in the bodyinterior incomplete and thus the size of the remaining Si₃N₄crystals/grains is typically fine and small. As a whole, Si₃N₄ in theinterior portion of the SiC—Si₃N₄ sintered body containing thesuperfines of SiO₂ is at least partially oxidized and thus more Si₂N₂Obiproduct is produced. The body has enhanced physical and mechanicalproperties. A body produced similarly to the P3 sample was thermallycycled in a tunnel kiln 220 times without cracking, a longer life thantypical for SiO₂ bodies under the same conditions.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character. It is understood that theembodiments have been shown and described in the foregoing specificationin satisfaction of the best mode and enablement requirements. It isunderstood that one of ordinary skill in the art could readily make anigh-infinite number of insubstantial changes and modifications to theabove-described embodiments and that it would be impractical to attemptto describe all such embodiment variations in the present specification.Accordingly, it is understood that all changes and modifications thatcome within the spirit of the invention are desired to be protected.

1. A substantially dense sintered body comprising: silicon carbidegrains; a SiO₂/Si₂N₂O matrix; and silicon nitride grains substantiallyevenly distributed throughout the SiO₂/Si₂N₂O matrix; wherein thesilicon nitride grains are substantially finer than the silicon carbidegrains; wherein the silicon carbide grains are substantially evenlydistributed throughout the SiO₂/Si₂N₂O matrix; wherein the siliconcarbide grains have a substantially narrow particle size distribution;and wherein the silicon nitride grains are disposed within respectivethin substantially silicon dioxide outer shells.
 2. A sintered body,comprising: a matrix including a mixture of SiO₂ and Si₂N₂O; a pluralityof SiC grains dispersed in the matrix; and a plurality of Si₃N₄ grainsdispersed in the matrix; wherein substantially all of the SiC and Si₃N₄grains are disposed within respective vitreous shells.
 3. The sinteredbody of claim 2 wherein the respective vitreous shells are substantiallySiO₂.
 4. The sintered body of claim 2 wherein the respective vitreousshells are substantially a mixture of SiO₂ and Si₂N₂O.
 5. The sinteredbody of claim 2 and further including metallic Si dispersed in thematrix.
 6. The sintered body of claim 5 wherein the metallic Si ispresent on the surfaces of at least some of the SiC grains.
 7. Thesintered body of claim 5 wherein the Si is present in at least some ofthe SiC grains.
 8. The sintered body of claim 2 wherein at least some ofthe Si₂N₂O is crystallized.
 9. The sintered body of claim 8 wherein atleast some of the Si₂N₂O has the form of crystals disposed on thesurfaces of Si₃N₄ grains.
 10. A densified body, comprising: asubstantially silica matrix; a plurality of SiC grains dispersed in thematrix; a plurality of Si₃N₄ grains dispersed in the matrix; and aplurality of Si₂N₂O crystals; wherein substantially all of the SiC andSi₃N₄ grains are disposed within respective substantially silica shells.11. The densified body of claim 10 wherein the respective substantiallysilica shells are substantially vitreous.
 12. The densified body ofclaim 10 wherein the Si₂N₂O crystals are generally located on thesurfaces of the Si₃N₄ grains.
 13. The densified body of claim 10 andfurther comprising metallic silicon distributed throughout the densifiedbody.
 14. The densified body of claim 13 wherein at least some of theSIC grains include voids and wherein the metallic silicon is disposedwithin at least some of the voids.